There is an increasing need for aligning features, such as features produced using photolithographic techniques, which features are to be positioned on opposite surfaces of a substrate. Such alignment can be required, for example, in manufacturing integrated circuits or optical elements, or combinations thereof. Current techniques for providing such alignment generally use one of two alternative techniques.
A first technique relies on the transparency of the substrates involved at the wavelengths used for alignment so that the alignment equipment in effect "looks through" the substrate, from one surface to the other, to align the oppositely disposed features thereon. In some cases, where the substrate is opaque in the visible part of the spectrum but is transparent in the infrared portion of the spectrum, the alignment equipment may be designed to use infrared wavelength signals. However, in such cases there is a loss in alignment precision because of the longer wavelengths involved and because the devices used therein are not optimized for operation at the longer wavelenths. This technique cannot be used in cases where the substrate is opaque in the portions of the spectrum that can be used for aligning (generally the visible or near infra-red).
A second technique, that is sometimes used, exposes the lithography simultaneously on both opposing surfaces. The equipment used therefor utilizes a mask for each surface, with the two masks accurately aligned before the substrate is introduced. This approach can handle opaque substrates, but the equipment is complex and expensive. In addition, it is restricted only to situations where it is practical to initiate the lithography on both surfaces simultaneously.
It is desirable then to develop a simple and relatively inexpensive technique which will provide alignment of oppositely disposed features using wavelengths, preferably in the visible part of the spectrum, whether or not the substrate is transparent or opaque at such visible wavelenths, and which allows lithography on one surface of a substrate to be aligned with existing lithography on the opposite surface.